A “shield” for unitary-lens eyewear or helmets consists of a single lens. In general, such a “shield” is formed from a transparent material as a single piece, that is a one-piece shield.
Typically, such one-piece shields are configured to curve around the eye to the side of the head (“wrap”) and/or tilt inward toward the cheekbone (pantoscopic tilt).
The explanation of this invention will be facilitated by defining some terms used in the following.
A spherical surface is a part of the inside or outside surface of a sphere. A cylindrical surface is a part of the inside or outside surface of a cylinder. A toroidal surface is a surface having mutually perpendicular principal meridians of unequal curvature, of which the cross-section in both principal meridians is nominally circular. An aspherical surface is a part of a surface of revolution having continuously variable curvature from the vertex to the periphery. An atoroidal surface is a surface having mutually perpendicular principal meridians of unequal curvature, of which the cross-section in at least one principal meridian is not circular. Principal meridians of a surface are those meridians of a surface which show the maximum and minimum curvatures on measurement. A progressive surface is a surface, which is non-rotationally symmetrical, with a continuous change of curvature over part or all of the surface, generally intended to provide increasing addition or degression power.
A freeform surface distinguishes from the above spherical, cylindrical, toroidal, aspherical and atoroidal surfaces. A freeform surface is a surface without symmetry over an area. Progressive surfaces as defined above having in addition in particular no mirror symmetry may be freeform surfaces. Most computerized modelling systems today use non-uniform rational B-spline (NURBS) mathematics to describe the surface forms; however, there are other methods such as bicubic splines or Gorden surfaces or Coons surfaces.
A plano lens is a lens with nominally zero dioptric power. A spherical lens is a lens with two spherical surfaces. A cylindrical lens is a lens with at least one cylindrical surface. A toric lens is a lens with at least one toroidal surface. An aspheric lens is a lens with at least one aspherical surface. An atoric lens is a lens with at least one atoroidal surface.
An ophthalmic lens is a lens intended to be used for purposes of measurement, correction and/or protection of the eye, or for changing its appearance. A spectacle lens is an ophthalmic lens worn in front of, but not in contact with, the eyeball. A corrective lens is a spectacle lens with dioptric power. A non-corrective lens is a spectacle lens with no dioptric power or such low dioptric power that it is nominally not used for corrective purposes.
The front surface of a spectacle lens is that surface of the spectacle lens intended to be fitted away from the eye. Accordingly, the back surface of a spectacle lens is that surface of the spectacle lens intended to be fitted nearer to the eye.
Focal power is a general term comprising the spherical and astigmatic vertex powers of a spectacle lens. Back vertex power is the reciprocal of the paraxial back vertex focal length measured in meters. Spherical power is a value of the back vertex power of a spherical-power lens or the vertex power in one of the two principal meridians of an astigmatic-power lens, depending on the principal meridian chosen for reference.
Prismatic deviation is the change in direction imposed on a ray of light as a result of refraction. Prismatic effect is the collective name for the prismatic deviation and base setting (that is the setting position for the prism base). Prismatic power is the prism value of the prismatic effect at the design reference point.
Dioptric power is a general term comprising the focal power and the prismatic power of a spectacle lens.
Optical axis is a straight line, perpendicular to both optical surfaces of a spectacle lens, along which light can pass undeviated. Vertex is the point of intersection of the optical axis with a surface of a lens. Therefore, back vertex is the point of intersection of the optical axis with the back surface of a lens.
The line of sight is the line joining the center of the fovea to the center of the exit pupil of the eye, and its continuation from the center of the entrance pupil forward into object space.
The normal line of sight is a fixed line that projects forward from the eye along the line extending straight ahead of the eye in the primary position with the head looking straight ahead. The line of sight is not normally understood to vary in a given individual. However the normal line of sight may vary (both horizontally and vertically) between individuals, because of variations of head and face morphologies (such as the distance between the eyes, and the location of the nasion and ears) which determine an as worn orientation of eyewear. Moreover, the normal line of sight may vary vertically between the right and left eye of a given individual, because of facial asymmetry. The “normal” line of sight is therefore often determined on a standardized head form, such as the Alderson head form, or the more current and accurate Canadian head form, in which a statistically average position of a line of sight has been determined.
A visual point is a point of intersection of the line of sight with the back surface of a lens. The distance visual point is the assumed position of the visual point on a lens, which is used for distance vision under given conditions. This is usually assumed to be the intersection of the line of sight with the lens, the eyes being in the primary position with the head erect.
The back vertex distance is the distance between the back surface of the lens and the apex of the cornea, measured with the line of sight perpendicular to the plane of the spectacle front.
The main fixation direction is the most common direction of the line of sight relative to the primary position.
Primary position is the position of the eye of a human relative to the head, looking straight ahead at an object at eye level. Monocular pupillary distance is the distance between the center of the pupil and the mid-line of the bridge of the nose or the spectacle frame when the eye is in the primary position.
The “as-worn” pantoscopic angle is the angle in the vertical plane between the normal to the front surface of the spectacle lens at its boxed center and the line of sight of the eye in the primary position.
Lateral wrap is the curvature or twist of a spectacle lens around the eye to the side of the head. The wrap angle, also known as face form angle or panoramic angle is the angle between the plane of the spectacle front and the plane of the right lens shape, or of the left lens shape. The right or left face form angle is regarded as positive if the temporal side of the right or left lens plane is closer to the head than the plane of the spectacle front.
A “nasal” direction is generally toward the nose, and a “temporal” direction is generally toward the temple. A “superior” direction is generally upward and an “inferior” direction is generally downward.
A lens produces a linear displacement, or foreshortening, of an image if the image is viewed along a direction of gaze that is not along the optical axis of the lens nor along the normal to the surface of the lens.
Prismatic deviation likewise may be induced if the direction of gaze is not parallel to the optical axis, regardless of where on the lens the direction of gaze intersects the surface. When the direction of gaze is not coincident with the optical axis of a lens, the lens will typically produce a total deviation, which is a combination of foreshortening and prismatic deviation.
Conventionally, the amount of the prismatic deviation is measured in prism diopters (PD or D).
One prism diopter is the unit of prismatic deviation, equal to 100 tan δ, where δ is the angle of deviation, in degrees (°). The prism diopter is a deviation measured in centimeters at a distance measured in meters. Prism diopters can therefore also be expressed in centimeters per meter (cm/m). The decentration can be horizontal, vertical, or oblique, but is generally evaluated in terms of horizontal and vertical deviations. A horizontal decentration of a non-plano lens with respect to an eye generally produces a horizontal prismatic deviation. A nasal decentration of a positive power lens produces a prismatic deviation that is referred to as “base-in” prism. Similarly, a temporal decentration of a positive power lens produces a prismatic deviation referred to as “base-out” prism. Nasal and temporal decentrations of minus power lenses produce base-out and base-in prism, respectively.
To compensate for horizontal prism in eyewear, the eyes must rotate horizontally by angles approximately equal to the prismatic deviations. If the prismatic deviations for both eyes have the same magnitude and direction, the normal line of sight is deviated, but the eyes move in a so called “yoked” alignment. If the prismatic deviations differ in magnitude or direction, a relative motion of an eye or eyes toward (convergence) or away from each other (divergence) is required to avoid diplopia (double vision). The differences in prismatic deviation thus give rise to a disjunctive or vergence demand that is quantified as the net prismatic deviation obtained by combining the individual prismatic deviations. The vergence demand can require either a convergence or a divergence of the eyes, but is referred to as a vergence demand in either case. Wearers are more comfortable if the yoked and vergence demands are kept small in order to permit accurate spatial perception and anticipation timing, and to avoid eye fatigue.
The vergence resulting from prismatic deviations for both eyes depends on both the magnitude and direction of the prismatic deviations.
If the amount of prism induced for each eye is the same, the eyes will move together in a “yoked” rotation. If the amount of prism for each eye is not equal, then an additional vergence demand is imposed on the eyes, in which there must be relative movement of one or both of the eyes toward (convergence) or away (divergence) from each other. Such vergence is often incomplete, which can result in diplopia or poor perception. Even if the vergence is complete, it induces oculomotor strain that is uncomfortable for the wearer.
Vertical prism effects are generally divided into base-up and base-down prism. The same problems discussed with respect to base-out and base-in prism apply to vertical prism. Differences in vertical prism are not well tolerated, but “yoking” type prism, the same for both eyes, are well tolerated.
The amount of horizontal prism can vary across the lens, and imbalance can become more of a problem peripherally, where one eye is looking through a nasal portion of a lens while the other eye is looking through a temporal portion of the lens. The amount of vertical prism can also vary across the lens in a similar fashion when the eye is looking through a superior or inferior portion of the lens. This variation can create inaccuracies in visual perception across the field of view that are difficult to compensate, and are troublesome in recreational or sporting activities that demand accurate visual input.
There are a plurality of patents, patent applications and other documents dealing with shape and arrangement of one-piece shields in front of wearer's eyes, the respective influence on aesthetic aspects as well as the respective resulting optical properties and impact on wearer's visual impression. Some of these documents are herewith presented in the following.
U.S. Pat. No. 4,859,048 discloses a cylindrical lens for use in a pair of sunglasses, comprising a unitary pane of transparent material curved about an axis and having a substantially constant radius such that the lens defines a portion of the wall of a cylinder. The lens covers both eyes of the wearer and effectively shields the eyes from peripheral as well as direct bright light. The lens may have either a uniform thickness throughout, or may taper from a greater thickness in a region centered about the midpoint, generally above the nose of a wearer, to a lesser thickness near the peripheral ends of the lens. The unitary lens has an upper edge and a lower edge, whereby the lower edge has a nosepiece opening formed therein for mounting the lens on the nose of a wearer.
U.S. Pat. No. 5,774,201 discloses a lens for unitary-lens eyewear. The lens has an outer, convex surface, and an inner, concave surface, and a thickness therebetween. At least one of the outer, convex surface and the inner, concave surface has an arcuate cross-sectional configuration conforming substantially to an ellipse having an eccentricity. The lens may have any of a variety of configurations in the vertical planes, independent of the horizontal elliptical shape. Additionally, the lens may be of uniform thickness or of tapering thickness from a relatively thicker medial portion to thinner lateral portions. The lens has an upper edge and a lower edge, and the lower edge has a nosepiece opening formed therein for mounting the lens on the nose of a wearer. Such lenses do not comply with contemporary aesthetic requirements.
Therefore, nowadays, the surfaces of conventional shields are typically spherical or toroidal, that is, they have circular horizontal and vertical cross-sections at the center. Such a shield must be “tapered” if it is to have zero optical power. A one-piece shield like this with zero optical power, that is a toroidal plano lens, automatically has zero prism imbalance between the two eyes. The optical properties of such shields do only comply with present requirements in a specific arrangement in front of a wearer's eyes. In particular, if such lenses are wrapped and oriented with tilt, a wearer's perception is distorted.
U.S. Pat. No. 6,010,217 discloses an optically corrected shield for unitary lens eyeglasses or safety helmets. The preferred lens (shield) geometry may be either spherical or toroidal. In particular, at least the front surface of the shield conforms either to a portion of the surface of a sphere or a portion of the surface of a toroid. The shield has a front surface which conforms in a vertical plane to a portion of a first circle having a first center and the shield has a rear surface which conforms in the vertical plane to a portion of a second circle having a second center. The first and second centers are non-coincident, and lie on an optical axis which extends through the shield. The lens is oriented on the head of the wearer by a frame or helmet that provides both wrap and pantoscopic tilt but maintains the lens in a position such that the optical axis is maintained substantially in parallel to the normal sight line of the wearer. The parallel relationship between the optical centerline and normal line of sight was found to be partially successful in minimizing optical distortion caused by wrap and pantoscopic tilt, but these lenses still had undesired peripheral performance, with prismatic effects that produced yoked and vergence demands.
The document outlines that instead of spherical or toroidal front and back surfaces other lens geometries such as elliptical or aspheric may also be utilized. However, a detailed description of such lens geometries is missing in this document.
U.S. Pat. No. 6,129,435 disclose non-corrective protective eyewear with lateral wrap and pantoscopic tilt comprising lenses having an optical axis that is deviated away from the line of sight, in a direction generally opposite the inward tilt of the lateral wrap (horizontal tilt) and/or the incline of pantoscopic tilt (vertical tilt), to offset the tilt induced (horizontal and vertical) prism (see in particular FIGS. 11 and 12 and the explanation given therein). In particular the optical axis is angularly deviated at a sufficient angle away from parallel with the line of sight to minimize prismatic distortion, both along a line of sight and peripherally in the field of view.
According to the teaching of the above publications, low power may be introduced into the lenses to decrease their taper, further offset the tilt induced prism and astigmatism (particularly in peripheral fields of view), lessen weight, provide better physical stability, and allow more uniform light transmission than plano lenses. The document outlines that prism by tilt can be reduced by one or more of a combination of parameters, such as increasing the angle of deviation between the line of sight and optical axis, increasing the minus power of the lens, or reducing the base curvature of the lens. According to U.S. Pat. No. 6,129,435 the lenses having such parameters may be spherical, cylindrical, toroidal, elliptical, or of other not further specified configurations.
U.S. Pat. No. 6,454,408 B1 discloses an optical lens element being, for example, adapted for mounting in a frame of the shield type including first and second surfaces of complementary curvature. At least one surface exhibits a deviation in curvature from a standard optical surface of spherical or toric shape along the horizontal meridian inducing optical distortions such as astigmatism of more than 1.0 D. The first and second surfaces in combination define an optical zone exhibiting mean through power along at least one meridian being constant within ±0.25 D. This document discloses that the curvatures of the first and second surfaces may be smoothly varying functions that allow the surfaces of the optical lens element to deviate substantially from, for example, a conventional conic section whilst providing between them constant mean through power within ±0.25 D through the lens. That is, the surfaces of the optical lens element are disclosed as being asymmetric.
U.S. Pat. No. 6,364,481 B1 discloses in particular plano lenses for use in glasses of the wrap-around or shield type. The lenses may include a spherical, an aspheric, a toric, an atoric surface or any combination thereof or any other complex form and may exhibit an astigmatic correction. The lenses comprise a peripheral temporal zone which includes a prismatic correction to improve the overall field of vision of the wearer. The front and/or back surface of the optical lenses may further include a surface correction to at least partially compensate for prismatic errors in the primary line of sight (the zone of ‘straight-ahead’ vision). The surface correction may be a prismatic correction, in particular a base-in or base-nasal correction applied to the front and/or back surface.
Two further approaches of unitary eyewear to improve visual performance for the wearer known from prior art are described in the following with reference to FIGS. 1A, 1B, 2A and 2B. FIG. 1A shows a perspective view of a first example of non-corrective unitary lens eyeglasses 100 with a one-piece shield 102 and a frame 104 supporting the shield 102. FIG. 1B shows a horizontal cross-section of the shield 102 in a plane above the nosepiece opening 106. FIG. 2A shows a perspective view of a second example of non-corrective unitary lens eyeglasses 200 with a one-piece shield 202 and a frame 204 supporting the shield 202. FIG. 2B shows a horizontal cross-section of the shield 202 in a plane above the nosepiece opening 206.
If a spherical or toroidal shield has a single optical axis, and zero back vertex power, it will also have zero prism as worn when looking parallel to the optical axis. However, this zero prism criterion is not fulfilled everywhere for the rotating eye at any specific pupil distance. If the shield has two separate optical axes, one for each eye, then wrap and curvature can be decoupled. However, in the case the two halves of the shield would not meet smoothly in the center. FIGS. 1A, 1B, 2A and 2B show lens eyeglasses 100, 200 having such shields 102, 202 consisting of two halves each having its own optical axis. The shield-halves 102a, 102b and 202a, 202b, respectively, the shields 102 and 202 are composed of, each are spherical or toroidal, and each have a separate optical axis which is parallel to the line of sight. Each of these shield-halves 102a, 102b and 202a, 202b are tapered toward the temples. The shield 202 according to the second example comprises a broad not-optically-corrected feature 208 above the nose piece 206 instead of purely “butting” the two shield-halves 102a, 102b up against each other. The individual shield-halves 202a, 202b in these safety goggles 200 are also spherical or toroidal.
Due to cosmetic reasons the non-corrective unitary lens eyeglasses 100, 200 shown in FIGS. 1A, 1B, 2A and 2B may not be used for all purposes. Therefore, alternative solutions are required in order to fulfill both aesthetic and optical needs.
Becken et al: “Brillengläser im Sport: Optimierung der Abbildungseigenschaften unter physiologischen Aspekten”, Zeitschrift für medizinische Physik, Urban and Fischer, Jena, Deutschland, vol. 17, no. 1, of May 3, 2007, pages 56-66 discloses individualized mathematical optimization procedures for corrective sports spectacle lenses.
U.S. 2006/0098161 A1 discloses a unitary single lens or shield. The shield has left and right lens portions, respectively, each having a visual center positioned in the line of sight of the left and right eyes of the wearer in the as worn condition. In this regard, each of the lens portions is individually constructed. Each lens portion has a visual center, a central area, and a peripheral area, and both the convex and concave sides of the lens are configured accordingly. The inner concave surface of each portion may be defined as an aspheric NURBS surface. The document discloses to improve peripheral vision in the case of spherical lenses, cylindrical and toric lenses, and as an extension of the invention may be applied to any shape (free form).
This document also discloses a method of manufacturing a non-corrective optical lens blank adapted for mounting in eyewear after appropriate glazing, the method comprising the steps of: configuring an outer convex surface of the lens blank; configuring an inner concave surface of the lens blank; defining a reference axis relative to the outer convex surface; defining a visual axis relative to the reference axis; defining a visual area surrounding the visual axis, the visual axis showing the location where the visual area is intended in the as worn position; and modifying the inner concave surface such so as to improve optical quality of the lens such whereby the modified inner concave surface has continuous horizontal and vertical curvatures in both horizontal and vertical meridians, but of varying dimension. The method will not modify the general torical shape of the lens, but rather only one or both of the surfaces in such a way that the general Gullstrand shape is not changed.
Despite the unitary single lens or shield disclosed in this document having proven its worth, there is a need of further improvement.